The present invention relates in general to the control of kilns used for drying wood, and in particular to a new and useful method and apparatus for controlling a wood drying kiln which is based on changes of the shrinkage rate in one or more boards of the lumber charge. In accordance with the invention, this information is used to determine internal stress levels in the board which in turn can be used to identify the occurrence of peak stress, stress reversal and reduced shrinkage as the drying rate is reduced after an initial increase of the drying rate during each advancement of the kiln schedule.
Kilns have long been used to dry lumber, in particular hardwood, but also some softwoods, with multiple step schedules. It is also known to periodically change the internal conditions of temperature and humidity in a kiln, according to a manual or automated schedule for the purpose of maximizing the drying while minimizing damage to the lumber in case the moisture content is reduced too quickly.
This balancing of maximum drying rate and the need to avoid damage to the wood, is the subject of several patents and articles.
Presently kiln controls are based on a number of parameters such as electrical resistance of the lumber (U.S. Pat. No. 3,744,144), weight of the lumber (U.S. Pat. Nos. 1,593,890; 4,176,464; 5,226,241; 5,325,604), internal temperature of the lumber, air temperature decrease across the stack of lumber, or length of drying time. All are used to indicate the moisture content of the lumber. This inferred moisture content is an indirect and poor indicator of the internal stresses which are ultimately the key to drying efficiently while providing quality lumber. Further, all of the above methods have weaknesses which reduce accuracy in determining the true moisture content of the load.
Board shrinkage has also been examined by Fortin, et al. in 1994 for example, but it was stated that an abrupt change in slope in shrinkage curves that were used, were due to the occurrence of fiber saturation point (FSP). See Fortin, Y., M. Ilieva, A. Cloutier, and P. Laforest. 1994, "Potential use of a semi-ring extensiometer for continuous wood surface strain measurement during kiln drying." 4th IUFRO International Wood Drying Conference Aug. 9-13th, 1994 Roturua, New Zealand. Ed. by A. Haslett and F. Laytner, pages 329-336. The error in their conclusions were precipitated by the absence of any stress data collected and reliance on the traditional moisture content orientation of drying research. They also did not mention how the data could be used to automate the kiln process.
Fiber point saturation is not meaningful when considering average moisture content. It refers to a time when a cell wall in the wood contains the maximum amount of water but has no free water in the cell lumen. Stress reversal has been recorded to occur an at any board average moisture content between 60% and 30% (percentages in this disclosure are all by oven dry weight). The reason it occurred at about 33% for the researchers mentioned above is that they were using a particular schedule on a species which generally causes stress reversal to occur at about 30%. They did not realize that the abrupt change in slope they observed is caused directly by stress reversal, not moisture content. For this reason, the work of Fortin, et al. 1994, has not helped to progress automated kiln control.
Bello and Kubler (1975) developed a shrinkage verses fracture-strain theory based on the comparison of true surface shrinkage and fracture strain of the material. By knowing the experimentally determined average fracture strain of the material and temperature, a theoretical loss of moisture can be calculated whereby the shrinkage is less than the average fracture strain. When this moisture is lost, a new data set of moisture and temperature set points can be calculated to advance the schedule. A drawback to this theoretical system is that the kiln sample boards would still be used to monitor moisture loss. Another drawback to this method is the need to know beforehand the average fracture strain of the material which is variable from board to board, a reversion back to the traditional manual method. See Bello, E. and H. Kubler 1975 "Shrinkage-strain-control (S-S-C)--A new approach to the process of kiln-drying wood" Wood Science 7(3):191-197.
A second point mentioned in the Bello and Kubler article is shrinkage referred to in a paper by McMillen (1969). In the original paper, McMillen labels his graphs as shrinkage but refers to them in the caption as plastic strain (which they actually are) and not shrinkage. The curves are for released plastic strain of individual layers from a board, not an entire board or gross shrinkage as is measured by the present invention. The destructive, time-consuming method of slicing the board and measuring the released strain was only conceived as a research tool to measure stress gradients within a board and was never intended as a monitoring method.
Hill, in 1975, performed a study to measure "barreling" or "bulging" of the side edge of lumber to infer stress levels. He was never able to obtain repeatable results that could be used as a control device. See Hill, J., 1996, Personal communication, Apr. 26, 1995. Referring to the drawings, FIGS. 15, 16, 17, illustrate Hill's device. Hill also only sought to detection stress reversal, not peak stress nor reduced drying rates. Hill advocated a system which measures the moisture contain difference between the surface and center of the board to obtain a theoretical stress level. He assumed stresses develop after 30% moisture content has been reached. In contrast, shrinkage can develop as high as 60% moisture content. Hill's system thus is not an actual stress level monitoring device.
In Hill's device, a frame 1 includes a centrally located feeler mechanism 2 having a probe tip 3 for contacting the side edge of a board. The frame is held to the edge of the board by screws 5 and the differential between the longitudinal position of feeler 3 and fixed reference plate pins 4 measures the relative amount of bulging or cupping of the board edge.
Although Hill's system is a real-time system, the moisture stress gradient is based on moisture content and the differential shrinkage obtained by measuring the bulging and cupping at the edge of the board, is a strain measurement and does not reveal peak stress points in the board, which is a main consideration and preferred for the present invention.